This invention relates to a re-entrant circulating stream jet comminuting and classifying mill, and more particularly to that type of mill but with the jet streams redirected at an acute angle to the axis of the mill.
The present invention relates to comminuting and classifyng apparatus of the type disclosed in U.S. Pat. No. 2,032,827 issued Mar. 3, 1936 to Norwood H. Andrews for Method And Apparatus For Providing Material In Finely Divided Form. The comminuting principle disclosed in that patent, with various modifications and improvements, is probably the world's most universally used device for extremely fine dry grinding. Because only the energizing gas and the material under reduction moves within the apparatus, proper correlation between the two is critical to successful comminution of various materials. These relationships have been worked out and are known to persons who operate grinding mills of this type.
Since its introduction, there have been many studies of the action in this type of mill (also known as the Micronizer). Perry's Chemical Engineering Handbook, 5th Edition, page 43, Section 8, states that "[t]he action in these mills has been studied photographically and mathematically (Rumpf Chem. Ingr.-Tech. 32(3) 129-135, (5) 335-342 (1960) A.T.S. Translations (668 G 844 GJ)."
All of these studies confirm the fact that as material is fed into the mill, the angle of the jet streams discharged from a circular peripheral manifold initiates a rotation of the material in the mill until there is a turbulent circulating band of material through which the jet streams penetrate. Grinding or comminuting is the result of the jet streams entraining some of this circulating material at their discharge ports and projecting it at high velocity through the slower circulating load of material with the impact and attrition doing the grinding.
More grinding is done when the circulating load has considerable radial depth, but the increased mass of the circulating load removes energy from the rotation of the gas leaving less energy to drive the classifying vortex. Therefore a balance has to be maintained between the velocity of the classifying vortex and the amount of material fed into and circulating with the load. Thus, the radial depth and mass of the circulating load cannot be increased beyond certain limits without reducing the angular velocity of the vortex and thereby adversely affecting its ability to classify fine particles.
In the mid-1930's, this inventor had motion pictures taken of the operation of a Micronizer re-entrant circulating stream jet mill viewed through a glass top and bottom wall. These pictures showed that grinding is done as the jets pick up material at the peripheral wall (through aspiration) and project such material against material into the circulating load. These pictures also showed that the jet streams carry a considerable amount of material into the body of the classifying vortex, but very little grinding results from the impact of this projected material with material projected by the other jets in the mill. However, it was observed that struck material particles at the tangent circle, that is the imaginary circle to which all of the jet streams are tangent, more frequently glanced in a generally radial inward direction than axially. These particles probably move in this direction because this is the general component of force of the jet streams as they intersect with each other.
Because of this tendency of the materials to glance radially inward, it has been accepted practice to design re-entrant circulating stream jet comminuting and classifying mills with the tangent circle generally at the midpoint of the radial distance between the peripheral wall and the axial outlet. See, for example, the relative position of the tangent circle in relationship to the collector and peripheral wall in FIG. 2 of U.S. Pat. No. 2,032,827. See also the relative position of the product outlet in FIG. 1 of U.S. Pat. No. 4,056,233. The reason for positioning the tangent circle midway between the peripheral wall and the axial outlet mills is understood by reference to U.S. Pat. No. 2,032,827 at page 6, column 1, lines 59 to 74 which describes the relationship between the grinding zone and the classifying vortex, and is expanded upon in U.S. Pat. No. 4,018,388 at column 1, lines 60 through column 2, line 6. As noted in this latter patent, the tangent circle is kept radially outward from the outlet by a "safety distance factor" so that oversized particles can be returned from the classification zone to the grinding zone by allowing them to gain sufficient centrifugal momentum.
Because of this tendency in re-entrant circulating stream jet mills to project oversized particles into the classifying vortex, the prior art includes a fair amount of literature describing ways to eliminate "kickbacks" (oversized particles) in the final product. See U.S. Pat. No. 2,562,753 at column 1, lines 15 to 25; U.S. Pat. No. 2,690,880 at column 1, lines 15 to 34; U.S. Pat. No. 3,425,638 at column 1, lines 67 to 72; U.S. Pat. No. 3,559,895 at column 2, lines 50 to 60; and U.S. Pat. No. 4,056,233 at column 1, lines 24 to 35. The last mentioned patent specifically incorporates a physical barrier "for inhibiting premature discharge from the mill of those heavier and coarser particles of the material which per chance may have traversed the gaseous vortex without having been reduced to the desired classification size and effecting their re-entry into the vortex to further subject them to the grinding energy of the gaseous fluid until they have been reduced in size by attrition to their desired classification for ultimate discharge from the mill". It is interesting to note that even though the apparatus described in U.S. Pat. No. 4,056,233 positions the nozzles substantially midway between the peripheral wall and the product outlet, it is still considered necessary to incorporate a physical barrier to inhibit the premature discharge of oversized particles.
It is known that grinding in re-entrant circulating stream jet mills can be improved by pointing the jet stream in a more radial direction; that is by positioning the tangent circle nearer the outlet. See U.S. Pat. No. 4,018,388 at column 1, lines 62 to 64. But the above-mentioned "safety distance factor" established the general practice of operating re-entrant circulating stream jet mills by positioning the nozzles to direct their jet streams tangent to a circle substantially midway between the periphery and the outlet.
It is therefore an object of the present invention to provide a more finely ground product in a re-entrant circulating stream jet comminuting mill by redirecting the gaseous fluid jet streams to bring the tangent circle radially closer to the outlet without increasing the quantity of oversize particles in the final product.